**Question:** Following drug inhibits release of calcium from sarcoplasmic reticulum -

A. Verapamil
B. Diltiazem
C. Nicorandil
D. L-type calcium channel blocker

**Core Concept:**

Sarcoplasmic reticulum (SR) is an essential organelle in muscle cells, primarily involved in the regulation of calcium ions (Ca²⁺) concentration within the cell. Calcium ions play a crucial role in muscle contraction and relaxation. The release of calcium ions from SR is facilitated by the ryanodine receptor (RyR), a calcium release channel.

**Why the Correct Answer is Right:**

The correct answer, Diltiazem (a.k.a. Tiazac, Cardizem), is a calcium channel blocker that primarily inhibits L-type calcium channels. By blocking these channels, diltiazem prevents calcium ions from entering muscle cells, thus reducing the intracellular calcium concentration. Consequently, calcium release from the sarcoplasmic reticulum is hindered, leading to decreased muscle contraction and relaxation.

**Why Each Wrong Option is Incorrect:**

A. Verapamil (a.k.a. Calan, Isoptin, Verelan) is also a calcium channel blocker, primarily inhibiting L-type calcium channels. However, it differs from diltiazem in pharmacokinetics, pharmacodynamics, and side-effect profile.

B. Nicorandil (a.k.a. Nicorette, Nicovent) is a nitrate drug used primarily for angina relief. It is a nitric oxide-donating drug that acts as both a nitric oxide donor and an ATP-sensitive potassium channel opener. It doesn't inhibit calcium channels and is not relevant to the question.

C. Diltiazem also inhibits sarcoplasmic reticulum calcium ATPase (SERCA), which is responsible for the uptake of calcium ions back into the sarcoplasmic reticulum. By inhibiting SERCA, diltiazem reduces calcium reuptake, leading to increased cytosolic calcium concentration. However, this option is incorrect because diltiazem primarily inhibits L-type calcium channels, as explained above, and not SERCA.

**Clinical Pearl:**

Understanding calcium channel blockers and their mechanisms of action is crucial for understanding various cardiovascular medications and their effects. This knowledge is essential for healthcare professionals, particularly for making appropriate therapeutic decisions in clinical practice.